add hook for providing region code
add hook for providing region code

refs FOO-2054
flag=none

TEST PLAN:
  1) Canvas.region_code is a method
  2) it returns nil in open source canvas
  3) it is successfully overridden/implemented
     for INST canvas in production

Change-Id: I1848b42baf38ce4967e5ad81f6b15b2780375940
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/266835
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms